In the forthcoming evolution of mobile cellular standards like Global System for Mobile communications (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) will be used. Furthermore, in order to have a smooth migration from existing cellular systems to a new high capacity high data rate system in existing radio spectrum, the new system has to be able to operate in a flexible bandwidth. A proposal for such a new flexible cellular system is Third Generation (3G) Long Term Evolution (3G LTE), in everyday speech LTE, which can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as multiple access technique, called Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported at the largest bandwidth. However, not only high rate services are expected to be used in 3G LTE, but also low rate services like voice. Since 3G LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over Internet Protocol (VoIP) will be the service carrying speech.
Another important aspect of LTE is the mobility function, hence synchronization symbols, cell search and antenna detection procedures are of major importance in order for the User Equipment (UE) to detect and synchronize with other cells.
FIG. 1 schematically illustrates the frame structure of LTE. The frame structure of LTE comprises a radio frame comprising ten sub-frames. Each sub-frame comprises two slots. The transmission can be described with a resource grid of sub-carriers and available OFDM symbols, as illustrated in. FIG. 2, which illustrates the example of normal cyclic prefix length. Each element in the resource grid is called a resource element (RE) and each RE corresponds to one complex-valued modulation symbol. The number of OFDM symbols per slot is seven for normal cyclic prefix length and six for extended cyclic prefix length. A basic scheduling unit is denoted a resource block. Thus, a resource block is defined as seven or six consecutive OFDM symbols in the time domain and twelve consecutive sub-carriers in the frequency domain.
To achieve high data rates, spatial division multiplexing of multiple data streams to a single user equipment (UE) may be provided. Two, and up to four transmit antennas can be used.
Within each downlink sub-frame, downlink control signalling is located in the first n OFDM symbols, where n is three or less. There is no mixing of control signalling and shared data in an OFDM symbol. Downlink control signalling can comprise a format indicator to indicate the number of OFDM symbols used for control in this sub-frame, scheduling control information, and acknowledgement indicator associated with uplink data transmission. The sub-frames also comprise reference symbols at specific locations in time and frequency of the grid for the respective transmit antennas.
A fundamental problem in LTE operation is to find out if a given known signal is sent in a given set of REs, or, which particular signal in a given set of possible signals that is sent in a given set of REs. This problem occurs for instance when a primary synchronization signal has been detected, and the UE needs to detect which of many possible secondary reference signals that is transmitted next to the primary one. Another instance of the problem occurs in so-called blind detection of a possible second (or third and fourth) eNode B transmit antenna port. In the presence of such an antenna port, certain known reference signals, t1-t4 for the antenna ports, respectively, are sent as indicated above, and may thus be used to detect the presence of a second, third or fourth antenna port by suitable processing of the signals received in these REs. The knowledge for the UE about the number of transmit antennas used is of major importance for good signal power measurements, i.e. mobility reasons, as well as for the possibility to decode the control channel, having different coding depending on the number of transmit antennas, once a handover to a new cell is needed.
Given is thus a set V of REs, or positions of ODFM symbols. Each element of V is a couple (i,k), where i is the time-index and k is the frequency-index of the RE in question. The question to answer is then either                (i) was some given signal ri,k, (i,k) εV, sent in the REs V, or        (ii) given a set S={r(i,k)p, (i,k)εV}, p=1, . . . , P, of signals, which of the signals in this set was sent in the REs V?        
The standard model for signal transmission in OFDM systems is yi,k=hi,ksi,k+ei,k, where si,k is the transmitted signal, hi,k is the channel coefficient, yi,k is the received signal and ei,k is noise, all in RE (i,k). All these quantities are complex numbers; the hi,k often considered as random variables from a time-frequency random process, i.e. random field, while the ei,k are usually considered as independent, across time and frequency, complex symmetric Gaussian random variables with some, in general unknown, variance σ2.
A straightforward approach to solve either of the above questions is to first compute an estimate ĥi,k of hi,k, then compute ŝi,k=ĥi,k1yi,k as an estimate of si,k and finally compute the sum
  Q  =            ∑                        (                      i            ,            k                    )                ∈        V              ⁢                  r                  i          ,          k                *            ⁢                        s          ^                          i          ,          k                    
where super-index * denotes complex conjugation. If the si,k are indeed equal to the ri,k, then the sum roughly equals
  Q  =            ∑                        (                      i            ,            k                    )                ∈        V              ⁢                                    r                      i            ,            k                                      2      up to some noise, and is thus large. A large value of Q thus indicates that the signals ri,k were in fact transmitted, i.e. answer question (i) above. If it is to be decided on one candidate from a set of signals, i.e. question (ii) above, the signal yielding the largest value of Q is picked. This approach is known as coherent correlation. A problem with this approach is the estimation of the channel coefficients hi,k. Typically it is assumed that hi,k is constant over some span of time and frequency, i.e. a time-frequency rectangle in the grid of REs. The estimate ĥi,k is then computed for (i,k) within this rectangle as an average of ratios yi,k/r*i,k, where the r*i,k are some known signals transmitted within the time-frequency rectangle and the indices (i′, k′) range over the REs within the rectangle where these known signals are sent; the average forming ĥi,k is hence computed over the same indices. The known signals r*i,k can be e.g. parts of the primary synchronization signal, when it has been detected, which is placed close to the secondary synchronization signal, or they can be reference signals from antenna port 0, which is always present. To compute ĥi,k over different indices (i,k), the rectangle moves along. In short, ĥi,k is computed as a local average in the time-frequency RE domain. Sometimes this is a weighted average, where REs close to (i,k) are given larger weights.
Now, since the hi,k arise from a random process, they are not constant over the time-frequency rectangle, and therefore, the local average forming ĥi,k will suffer from bias as an estimator. If the rectangle is chosen large, this bias will be large, in particular in case of large speed and thus Doppler spread, and large delay spreads. On the other hand, if the time-frequency rectangle is small, the variance of ĥi,k will become large because the average has few terms.
Another problem with coherent correlation is that if there is a frequency error in the local receiver's local oscillator, this will introduce a phase shift, Δ say, per RE time unit, i.e. OFDM symbol. This will in turn affect the value of Q above. Thus one often considers the absolute value |Q|, rather than Q itself.
Yet another problem with coherent correlation pertains to question (i) above; how large should Q, or |Q|, be in order to decide that the signals si,k were sent? The appropriate threshold depends on the transmission power and the noise level, of which at least the latter is not known exactly. Thus it is typically problematic to determine an appropriate threshold.
Thus, there is a need for at least one of an improved approach to determine if a given signal is sent in a given set of OFDM symbols, i.e. to answer question (i) above, and an improved approach to determine what signal that was sent, i.e. to answer question (ii) above.